Dispersal

Question 1

What is dispersal and why is it important?

Answer 1

Dispersal is any movement of individuals with potential consequences for gene flow across space. This includes both natal dispersal and breeding dispersal.

Understanding dispersal and its evolution is crucial to improve the management of natural populations.

1. Dispersal affects the distribution of genetic diversity through space by increasing the proportion of total diversity contained within rather than between populations
2. Dispersal can help mitigate the effect of drift in small populations, decrease mutation load, and therefore reduce the risk of extinction (but it can also increase the risk of simultaneous extinctions by increasing the synchrony of the dynamics of local populations)
3. Dispersal affects the evolution of speciation, inbreeding depression, cooperation and sociality, and many life-history traits
4. Dispersal plays a key role in community dynamics

Question 2

Is dispersal plastic?

Answer 2

Yes. Density affects emigration and immigration rates in animals and seed dispersal in plants, with effects carried over several generations

Conditional dispersal expression doesn’t reflect only the variation of constraints on the dispersal process, but also the great plasticity in the organism’s broad response to various environmental cues during dispersal

Question 3

How fast can dispersal evolve?

Answer 3

Rapid dispersal evolution requires both the presence of heritable genetic variation for traits affecting dispersal behaviour and strong selection acting on these traits.

The response of dispersal traits to artificial selection can be fast.

Evidence for the short-term evolution of dispersal in nature comes from specific situations, such as oceanic islands, variation in landscape fragmentation, ecological succession, and biological invasions

Question 4

How do we model selection on dispersal?

Answer 4

Selection on genotypes differing in their dispersal propensity is generally frequency dependent, because dispersal affects both the spatial distribution of genetic diversity and population dynamics and alters the selective environment for different genotypes

Models have aimed at predicting which mean dispersal phenotypes or coalition of different phenotypes would dominate in the long term - so game theory has been and still is the preferred approach to investigate dispersal evolution patterns

But model assumptions typically lack a great deal of realism - dispersal is treated as a fixed process with a constant fraction of the population dispersing each generation

Question 5

Does dispersal allow escape from competition?

Answer 5

Competition is at the core of many theoretical and empirical studies of dispersal, but need to distinguish the effects of competition with conspecifics in general from those of competition with relatives. Escaping conspecific competition is a major potential benefit of dispersal.

In landscapes with variable density through space, dispersal by simple diffusion results in a net flow of individuals from highly population to less populated regions.

It is not always obvious from empirical data that dispersal indeed allows escape from overcrowding. Dispersal agent behaviour, directed dispersal, and habitat selection may indeed often result in postdispersal aggregation of high density

Dispersal can be understood as an altruistic behaviour, providing no direct ecological benefit to the dispersed individual, but alleviating competition for its kin

Distinguishing between the relative effects of kin competition and demographic stochasticity on the evolution of dispersal is often not obvious. This is especially the case in models in which small finite local population sizes generate both strong local genetic relatedness and random variation in population characteristics

Interactions between kin selection and demographic stochasticity can also lead to counterintuitive emergent properties, such as the evolution of increasing dispersal rates with increasing dispersal costs

Question 6

Is dispersal an inbreeding avoidance strategy?

Answer 6

Sex-specific dispersal rates in animals and pollen dispersal in plants have often been interpreted as mechanisms for inbreeding avoidance, even though alternative explanations involving kin-competition avoidance can also explain the same patterns

Heterosis (higher fitness of progeny born to parents originating from different populations) is expected in populations in metapopulations with strong genetic structure, in which kin competition is also intense. Heterosis favours divergence in sex-specific dispersal rates, kin competition tends to have a stabilizing effect

Heterosis increases the effective migration rate, decreasing relatedness and weakening the incentive effects of kin competition on dispersal

Question 7

Is dispersal an adaptation to ephemeral habitats (short-lived habitats that only support a few generations)?

Answer 7

There are two main reasons why dispersal may provide adaptation to ephemeral habitats:

1. Dispersal allows tracking patches of favourable habitat and escape from deteriorating local conditions.

Predictable habitat deterioration through time creates additional selection pressures favouring dispersal because less dispersive genotypes tend to be more frequent in older patches of habitat of lesser quality.

Low conspecific density can convey different types of information about habitat quality, reflecting either low competition for resources of deteriorating ecological conditions

2. Dispersal acts as a bet-hedging strategy in temporally variable environments.

Genotypes with a higher dispersal ability better sample habitat variation within a generation, thus reducing the generation-to-generation variance in their mean performance

Question 8

How costly is dispersal?

Answer 8

Dispersal is a risky behaviour.

1. Mortality may be increased during the vagrancy stage of dispersal.
2. Dispersal is also risky when habitat selection during the settlement stage is constrained or limited, leading to frequent immigration into nonfavourable habitat.
3. Dispersal may also result in a loss of social status when joining a new group of individuals, involving an exposure to xenophobic behaviours or a loss of cooperation with related individuals

Dispersing individuals are generally not a random subset of the population and have behavioural, physiological, and morphological attributes to reduce mortality during dispersal and increase settlement success in new patches of habitat

Question 9

What are the different types of dispersal?

Answer 9

Natal dispersal - movement by which an individual leaves its birthplace to engage into mating or reproduction somewhere else

Breeding dispersal - movement between two reproduction events for the same individual

Question 10

What are the different stages of dispersal?

Answer 10

Departure (emigration)
Vagrant stage
Settling (immigration)

Question 11

What is local adaptation?

Answer 11

the higher fitness of resident genotypes in their native environment relative to that of immigrant genotypes in the same environment.

Local adaptation arises when different alleles have different effects on fitness in different environments

Question 12

What is a metapopulation?

Answer 12

a set of discrete populations connected by dispersal

Question 13

What is conditional dispersal?

Answer 13

an individual’s decision over whether to disperse is a response to environmental conditions

Question 14

What is game theory?

Answer 14

in the context of population genetics, theory seeking approximations for the long-term evolution of traits, when frequency dependence is expected

Question 15

What is relatedness?

Answer 15

a function of probabilities of genetic identity that measures the increased probability of recent coalescence between some pair of genes relative to another

Question 16

What is coalescence?

Answer 16

time in the past when two particular gene copies of the present population had their most recent common ancestor

Question 17

What is kin competition?

Answer 17

competition among individuals bearing the same allele

Question 18

What are some of the potential driving forces for the evolutionary causes of dispersal?

Answer 18

1. Kin competition
2. Inbreeding
3. Resource competition
4. Environmental stochasticity

Question 19

What are some ultimate causes of dispersal?

Answer 19

1. Kin interactions - Kin selection occurs when selection on individuals favours traits that increase the fitness of close relatives. Kin selection may favour dispersal as a mechanism to reduce competition between kin at the natal site - dispersal will result in siblings competing with non-siblings at many sites. For some animals, kin presence can have a positive effect through cooperation.
2. Inbreeding avoidance - The negative fitness consequences associated with breeding between close relatives are expected to be select for mechanisms to avoid such matings. Dispersal is predicted to be sex-biased to avoid inbreeding but asymmetries in the level of intrasexual kin competition may also lead to sex-biased dispersal. A dispersal strategy sensitive to the presence of opposite-sex kin would be suggestive of inbreeding avoidance.
   3a. Habitat variability - population dynamics. Variation in population dynamics can affect the realised patch quality, even if the intrinsic patch quality is constant. This arises from changes in population densities leading to changes in the social and competitive environment over time
   3b. Habitat variability - intrinsic patch quality. Theoretical studies agree that spatially uncorrelated, temporal variation in patch carrying capacity should select for dispersal, and equally, variation in individual demographic parameters caused by temporal environmental variation favours dispersal. Dispersal is selected against as the individuals with most to gain from dispersal are those occupying low-quality patches, but as low-quality patches contain relatively fewer individuals, dispersal is not selected on average.

When habitat loss leads to habitat fragmentation, both the costs and benefits of dispersal can be increased, specifically if the remaining fragments are more liable to extinction

Question 20

What are proximate causes of dispersal (for emigration)?

What influences the ability of an organism to emigrate and leave a habitat patch?

Answer 20

1. Density - Increasing population density can reduce individual fitness and become a driving force for dispersal, through two main types of competitive interactions: exploitative competition (density effects on per capita resources) and interference competition (direct effects on density itself). Different factors can operate at different densities, so a simple linear relationship between density and emigration may not be expected.
2. Food availability - Both the density of competitors and food levels together determine the per capita resources available for a population - so the effects of food availability are predicted to vary with population density
3. Interspecific interactions - Not only interactions with competitors, but also with other guilds (parasites/predators) can determine the suitability of a patch for an individual
4. Sex ratio - Sex ratio may influence dispersal if there is asymmetrical competition between males and females or if the mating success varies in space due to differences in the number of available mate
5. Relatedness - the kinship of interacting individuals can be a proximate mechanism triggering dispersal when dispersal has been selected as a means to avoid inbreeding or kin competition
6. Patch size - negative correlation to emigration rate - influences migration through edge to size ratios.
7. Matrix habitat - Costs paid during inter-patch movement comprise factors such as greater energy expenditure and exposure to predation and may vary according to the matrix habitat.
8. Patch isolation - The degree of isolation of a patch will strongly impact the cost of dispersal as costs of movement increase with travel time

Question 21

What is exploitative competition and interference competition?

Answer 21

Exploitative competition is density effects on per capita resources

Interference competition is direct effects on density itself

Question 22

What are proximate causes of dispersal (for inter-patch movement)? What influences the ability of an organism to move from one patch to another?

Answer 22

1. Matrix habitat - Movement of an organism can be strongly influenced by habitat type and fine-scale spatial heterogeneity. Movement rates can be different through different habitats
2. Search strategies - describing animal movement quantitatively can be useful in allowing comparison of movement behaviour under different conditions
3. Habitat cues - During inter-patch movement, the use of cues, identifying the direction and proximity of suitable habitat may reduced the search time and potentially increase dispersal success

Question 23

What are proximate causes of dispersal (for immigration)? What influences the ability of an organism to immigrate successfully to a new area?

Answer 23

1. Patch size - patch area is predicted to determine the probability that a patch will be located by a randomly dispersing animal as individuals are relatively more likely to find a larger patch than a small patch in the matrix
2. Isolation - As movement costs accumulate with distance moved, successful movement is expected to be greater as the distance between patches decreases. Patch isolation is also likely to play a role in determining the importance of other immigration correlates such as habitat cues
3. Habitat cues - Habitat cues used to orientate an animal towards a habitat patch whilst moving in the matrix can also be used in immigration decisions once a patch has been located. The presence of conspecifics can make a patch more detectable rather than reflecting or increasing patch quality

Question 24

What is the difference between patch finding and immigration?

Answer 24

Immigration is the choice of moving into and staying within a patch once it has been encountered, so the difference is choosing to stay within a patch once it has been found

Question 25

What are some potential causes of variation in individual dispersal propensity?

Answer 25

1. Sex - sex-biased dispersal is common in mammals (male-biased) and birds (female-biased) 2. Developmental stage - The relationship between age and dispersal propensity may be determined by differing constraints or costs of dispersal or differences in the pressures to disperse among age classes, or some combination of the two 3. Body size/condition - As the level of competition increases, smaller and competitively inferior individual may 'feel' the pressure first and display a greater dispersal propensity

Question 26

What is the rescue effect?

Answer 26

Immigration of individuals into an occupied patch can decrease extinction risks by simply increasing the size of the population

Question 27

What is the effect of positive density-dependence on dispersal?

Answer 27

Positive density-dependent immigration strategies result in higher density patches receiving more immigrants than small populations. There is an increase in the extinction rate and a reduction in the colonization rate, which leads to smaller metapopulation size and reduced viability of the system The opposite is true for negative density-dependence

Question 28

What are the four main mechanistic components of organismal movement?

Answer 28

1. the internal state (why move?) 2. motion (how to move?) 3. Navigation (when and where to move?) 4. external factors affecting movement

Question 29

What are the major existing movement research paradigms?

Answer 29

1. Biomechanical paradigm 2. Cognitive paradigm 3. Random paradigm 4. Optimality paradigm The biomechanical and cognitive paradigms do not usually consider movement patterns - instead focus on basic mechanisms underlying movements Random and optimality paradigms tend to overlook the mechanics - random focuses on the movement patterns, and optimality focuses on the interplay between the internal state and external factors

Question 30

What is the biomechanical paradigm of movement research?

Answer 30

Focuses on the physical machineries of motion for the individual, including their mechanics, energetics, and physiology. Provides a good description of the motion capacity of individuals but ignores the questions of why, where and when organisms move

Question 31

What is the cognitive paradigm of movement research?

Answer 31

Focuses on the navigation mechanisms to elucidate the rules by which individual motile organisms make movement-related decisions. Goes to the level of cells in the brain Pays less attention to how individuals implement decisions or how the resulting movement feeds back to the internal motivation to move

Question 32

What is the random paradigm of movement research?

Answer 32

This is the prevailing approach. Characterized by simple phenomenological descriptions of movement paths and by null models related to the theories of random walk, diffusion, and anomalous diffusion. Often used to analyze large-scale movements in landscape ecology

Question 33

What is the optimality paradigm of movement research?

Answer 33

Explores the relative efficacy of different strategies in optimizing some particular fitness currency (e.g. energy gain, survival, reproduction) over ecological or evolutionary time scales Principal concepts in this paradigm, such as the ideal free distribution and central-place foraging theory do not explicitly deal with movement per se and tend to neglect the constraints imposed by the limited motion and navigation capacities of individuals and the partial information they have about the environment

Question 34

What are some challenges to applying movement ecology frameworks?

Answer 34

Four main challenges: 1. Because of its inherent dependency on the sampling protocol and frequency, parsing the movement path into a string of elemental units is a major challenge 2. Path segments need to be classified in terms of the basic functional units of the lifetime track 3. Assessing how processes operating at multiple spatiotemporal scales determine the composition of movement phases and their frequency in an individual's lifetime track is challenging 4. Movement data must be placed into its proper environmental context

Question 35

What are ultimate and proximate causes of dispersal?

Answer 35

Proximate - aims to answer how it works. Deals with mechanisms (internal - physiological or genetic. external - environmental) Ultimate - aims to answer why it is the way it is